1. Field of the Invention
The present invention relates to laser microscopes.
This application is based on Japanese Patent Application No. 2007-032247, the content of which is incorporated herein by reference.
2. Description of Related Art
Laser scanning microscopes of the multiphoton excitation type are known in the related art. These microscopes irradiate a specimen with ultrashort-pulsed laser light with an average output power of several milliwatts to several hundred milliwatts and a pulse width of several tens of femtoseconds to several hundred femtoseconds, via an objective lens, and observe multiphoton fluorescence produced by a multiphoton-excitation effect induced at the focal plane of the objective lens.
In this type of laser-scanning microscope, it is preferable to introduce the laser light from the light source by using an optical fiber, which allows easy maneuverability and is highly convenient.
When ultrashort-pulsed laser light having a femtosecond-order pulse width is introduced into the optical fiber, the high optical density inside the fiber brings about a nonlinear effect (self-phase modulation: spectral broadening) which, in conjunction with the group velocity dispersion, causes the pulse width of the ultrashort-pulsed laser light emitted from the end of the objective lens to increase, resulting in the problem of impaired multiphoton-excitation efficiency.
One known way to solve this problem is to use a laser microscope in which a negative-dispersion optical system is disposed between the optical fiber and the laser light source emitting the ultrashort-pulsed laser light, and a positive-dispersion element is disposed between the optical fiber and the microscope main body (for example, see Japanese Unexamined Patent Application, Publication No. 2000-206415 and U.S. Pat. No. 6,269,206).
With this laser microscope, because the ultrashort-pulsed laser light is introduced into the optical fiber with its pulse width widened in advance by the negative-dispersion optical system, it is possible to avoid the generation of the nonlinear effect in the optical fiber.
Then, because recompression of the pulse width of the ultrashort-pulsed laser light is performed near the exit end of the optical fiber, to avoid generation of the nonlinear effect associated therewith, positive dispersion is applied by the positive-dispersion element disposed at the downstream side of the optical fiber to make the pulse width after exiting the positive-dispersion element substantially the same as that before being incident on the negative-dispersion optical system.
In the laser microscopes disclosed in Japanese Unexamined Patent Application, Publication No. 2000-206415 and U.S. Pat. No. 6,269,206, to prevent the pulse width of the ultrashort-pulsed laser light from expanding while propagating through the optical fiber, it is not possible to use a multimode fiber; instead, it is necessary to use a single-mode fiber. The core diameter of a single-mode fiber is generally about several micrometers. In order to suppress the generation of a nonlinear effect in the fiber, it is necessary to substantially increase the amount of negative dispersion to reduce the incident peak intensity at the fiber, and it is necessary to substantially increase the amount of positive dispersion of the positive-dispersion element located at the downstream side of the fiber.
However, when the amount of positive dispersion to be applied is too large, the thickness of the positive-dispersion element increases, and a drop in the output power due to internal absorption in the positive-dispersion element becomes more pronounced, which causes a reduction in the efficiency of the multiphoton-excitation effect in the specimen.
Moreover, when the amount of positive dispersion to be applied is too large, the size of the positive-dispersion element becomes large, thus making it difficult to attach it to the microscope main body and to accommodate it therein. When the microscope main body increases in size, the advantage of being able to improve the maneuverability of the microscope main body by using the optical fiber vanishes.